Method of controlling phosphor ph in processing crt

ABSTRACT

The pH of a phosphor is established at a desired nominal value by utilizing a sodium chloride-magnesium chloride flux material which results in optimum phosphor screen exposure time and optimum phosphor pattern adherence. The pH of a slurry containing a sodium chloride-magnesium chloride flux phosphor is maintained at a desired value by the addition of ammonium hydroxide to the slurry.

United States Patent [72] Inventors [54] METHOD OF CONTROLLING PHOSPHOR PH IN PROCESSING CRT 1 Claim, No Drawings [52] US. Cl 96/93, 96/36.1,117/33.5 C, 117/335 CM, 252/301.3 [51] Int. Cl G03c 1/66 [50] Field of Search ..96/93, 36.1, 75; 117/335 R, 33.5 C, 33.5 CM, 33.5 CS, 33.5 CP; 252/3013 [56] References Cited UNITED STATES PATENTS 2,184,310 12/1939 Meigs et a1 96/93 X 2,716,061 8/1955 Lupo 96/93 2,744,072 5/1956 Meister 252/3013 2,898,225 8/1959 Rychlewski et al. 1 17/335 3,342,594 .9/1967 Kaplan 96/361 3,287,151 11/1966 Bishop et a1. 117/335 OTHER REFERENCES Jorgensen et aL, The Sensitivity of Bichromated Coatings," 1954, p. 57- 58 Kosar, J., Light-Sensitive Systems," 1965, pp. 78- 80.

Primary Examiner-Charles I... Bowers, Jr. Attorneys-F. H. Henson, C. F. Renz and M. P. Lynch METHOD OF CONTROLLING PHOSPHOR PH IN PROCESSING CRT BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates in general to the processing of phosphor screens and more particularly to a method for controlling the pH of a phosphor material to establish optimum exposure time and pattern adherence during processing of screens on the faceplate panels used in shadow mask-type color cathode-ray tubes.

2. Description of the Prior Art In making phosphor screens using the slurry process, phosphor powder is mixed with a liquid e.g., water, to form a slurry, and a quantity of the slurry is deposited onto and spread over a faceplate panel by spinning and tilting the panel. In making mosaic-type screens, such screens being used in apertured shadow mask-type colored cathode-ray tubes, by the so-called'direct photographic process, a photoresist is also included in the slurry. The photoresist may, for example, comprise a polymer such as polyvinyl alcohol (PVA) as the binder which is mixed with a photosensitizer; e.g., a dichromate such as ammonium dichromate or potassium dichromate.

After a layer of the photosensitized slurry has been spread over the faceplate panel and dried, selected areas of the phosphor coated faceplate panel are exposed to a desired ultraviolet light pattern. The exposed panel is then developed by washing it to remove the unexposed areas. This process is repeated for each of a plurality of desired phosphors, such as red-emitting, green-emitting and blue-emitting phosphors.

For a detailed description of the photographic process refer to U.S. Pat. No. 3,406,068 issued to H. B. Law, Oct. 15, I968.

The basic factors controlling the time required to expose a color face panel are the color to be printed, and the diameter of the dot sizes required. The diameter of the dot is a direct function of the exposure time. In addition to affecting the dot size printed, the exposure time also directly affects the adherence of the printed dot to the surface of the face panel.

If the demands for color printed face panels were small and each face panel fabricated on individual basis, a controlled exposure time sufficient to print the required dot and insure good dot adherence could be tolerated.

Inasmuch as the demand for color printed face panels is such as to dictate optimum production schedules necessitating minimum exposure time without appreciably reducing the adherence characteristics of the printed pattern, the control of pattern exposure time is critical.

SUMMARY OF THE INVENTION It has been found that one of the major factors contributing to the amount of time required to expose a phosphor dot pattern on a face panel is the pH of the phosphor and the phosphor slurry. This relationship between pH and exposure time is particularly apparent in blue-emitting phosphor material and resultants slurries.

A blue-emitting phosphor slurry having a low pH exhibits minimum exposure time while sacrificing good dot adherence, while a phosphor slurry having a high pH exhibits good dot adherence but unacceptable exposure times.

The blue-emitting phosphor generally utilized in the fabrication of color phosphor screens is a zinc-sulfide, silver activated composition.

As is well-known in the art it is desirable to use a copper resistant blue-emitting phosphor to expose the blue dot pattern of a phosphor screen in order to prevent blue phosphor breakdown during subsequent heat cycles and to insure well defined tricolor face panel characteristics. The copper resistant blue phosphor resists copper contamination which is the blue pattern produces white.

The standard blue-emitting phosphor which incorporates a magnesium chloride flux exhibits a desired resistance to copper contamination but also exhibits an undesirably high pH resulting in lengthy exposure times.

The standard blue-emitting phosphor which incorporates a sodium chloride flux conversely exhibits a relatively low resistance to copper contamination but does exhibit a low pH and the accompanying desired low-exposure times.

The invention comprises in part a blue-emitting phosphor that incorporates a flux mixture of sodium chloride and magnesium chloride to produce a phosphor pH that exhibits a lowexposure time comparable to the noncopper resistant type but without exhibiting noticeable loss of copper resistance.

Furthermore it has been determined that the pH of a slurry containing a sodium chloride-magnesium chloride fluxed material can be adjusted and maintained at a desired pH value by the controlled addition of ammonium hydroxide to the slurry. DESCRIPTION OF THE PREFERRED EMBODIMENT The relationship between phosphor pH and phosphor screen exposure time was evaluated utilizing blue-emitting phosphors. It is apparent, however, that the discussion is concerned with fluxing of phosphors in general in which sodium chloride or magnesium chloride flux material is utilized.

Test results comparing noncopper resistant blue-emitting phosphor, that fluxed with sodium chloride, and copper resistant phosphor, that fluxed with magnesium chloride, showed that sodium chloride fluxed phosphor exhibited a pH of 6.2-6.5 with a 50 percent reduction in exposure time as compared to magnesium chloride fluxed phosphor which ex hibited a pH of 8.8-9.5.

Further testing indicated a pH range between 7.0 and 8.5 offered an acceptable compromise between the maximum adherence provided by the magnesium chloride flux and the minimum exposure time provided by the sodium chloride flux while exhibiting the desired resistance to copper contamination.

Analysis of a magnesium fluxed phosphor sensitized with unbuffered ammonium dichromate indicated an initial pH comparable to that of sodium chloride fluxed phosphor and exhibited comparable exposure time while retaining its copper resistant characteristics. This desirable condition deteriorates with time, however, due to leaching action of the magnesium on the phosphor into the slurry which results in the pH steadily increasing to the original pH value between 8.8 and 9.5. The leaching action is believed to be the result of free magnesium combining with water to form a buffer of magnesium hydroxide. Attempts to retard the leaching action and maintain a desirable pH value by the addition of a weak acid were not successful. 7

It is apparent that efforts to increase the pH of the sodium chloride fluxed phosphor would be ineffective inasmuch as the magnesium chloride required for the desired copper resistant characteristics would be absent.

Further evaluation of the desirable characteristics of the magnesium chloride fluxed phosphor and the sodium chloride fluxed phosphor, led to the investigation of a blue-emitting phosphor fluxed by a sodium chloride magnesium chloride mixture. 7

Blue-emitting phosphors incorporating a flux material comprised of various ratios of sodium chloride to magnesium chloride were evaluated and conclusively indicated that the pH of the blue phosphor could be established at a stable desired value.

It was further determined by experimentation that the pH of a sensitized phosphor slurry prepared with a phosphor incorporating a flux mixture of sodium chloride and magnesium chloride could be adjusted from a nominal minimum value, such as 6.8-7.0, established during phosphor manufacture, to an optimum pH value between 7.0 and 8.5. This range of pH value accounts for the pH values which reflect a suitable compromise between exposure time and pattern adherence while maintaining desired resistance to copper contamination.

The controlled addition of ammonium hydroxide to the slurry of blue-emitting phosphor fluxed by a sodium chloridemagnesium chloride mixture provides control adjustment of the slurry pH to a desired pH value of 7.0-8.5. This range of pH values results in exposure time which are approximately 40 emitting phosphor, a polyvinyl alcohol binder, a dichromate sensitizer, a magnesium chloride-sodium chloride fluxing compositions, and ammonium hydroxide of an amount sufficient to maintain the pH of said slurry composition between 7.0 and 8.5.

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